As is well known in the art, the successful implementation of optical communications systems requires devices capable of reliably controlling the propagation of light. Basic optical processing functions required in optical communications include, but are not limited to: optical modulation (which includes amplification and attenuation of optical power); phase control (delay); and switching. In Wavelength Division Multiplexed (WDM) and Dense Wavelength Division Multiplexed (DWDM) communications systems, significant channel power imbalance may be generated and channel equalization become necessary where the above noted basic functions must be performed on a per-channel basis.
In modern high performance optical communications systems, data rates of 10 GHz or more can be encountered on each channel. In addition, using known optical amplification techniques such as Raman pumping and Erbium Doped Fiber Amplification (EDFA), optical transmission spans of 1000 Km or more can readily be achieved.
As is known in the art, Raman pumping and EDFA typically do not produce a flat gain profile across the spectral window of interest. Typically, a flat gain response is obtained by the use of filters which impose wavelength-dependent losses to counterbalance gain variations of the EDFA. Gain profiles of high flatness can be achieved using this approach, but only within the specific design parameters of the amplifier, which are specified by customers to meet their particular link budgets.
However, in practice, the input signal level can vary in time (e.g., the number of wavelength channels and/or the optical power in one or more channels can change), which changes the gain profile of the EDFA. If the gain flattening filters are static, changes in the input signal level can produce “gain tilt” (i.e., different wavelengths will have different gain). When gain tilt is amplified by successive EDFAs along a network link, significant optical power variations across the transmission window can be produced at a receiver. This problem can be solved using variable optical attenuators (VOAs), which enable the design of filters having a controllable attenuation.
Various types of VOAs are known, using, for example, mechanical, thermal and electrical mechanisms of activation. For example, U.S. Pat. Nos. 5,966,493 and 6,370,312 (both to Wagoner et al.) teach tunable optical attenuators in which the cladding of an optical fiber is side-polished to expose a surface though which light propagating in the fiber core can escape. A controllable refractive index material is positioned against this surface. Changes in the refractive index of the controllable index material can be used to control the amount of light coupled out of the fiber core. U.S. Pat. No. 6,011,881 (Moslehi et al.) teaches a tunable optical filter in which the cladding of an optical fiber is side-polished in the vicinity of a Fiber Bragg Grating (FBG). A controllable refractive index material is positioned against this surface. Changes in the refractive index of the controllable material can be used to vary the effective refractive index of the core material, and thus the reflective wavelength λR of the FBG.
U.S. Pat. No. 4,986,624 (Sorin et al.) teaches an optical fiber evanescent grating reflector, in which the cladding of an optical fiber is side-polished to expose a surface that is penetrated by the evanescent field of light propagating in the fiber core. A grating placed on the exposed surface causes reflection of light at a selected wavelength. The amount of optical energy reflected is a function of the strength of interaction between the evanescent field of guided modes and the reflection grating, and thus can be controlled by varying the distance between the reflection grating and the exposed surface.
Because the above devices require a side-polished fiber, the optical properties of which are also highly dependent on the radius of curvature, the effective length of the polished zone and the distance between the exposed surface and the fiber core, they tend to be difficult to manufacture. They also tend to be highly sensitive to temperature and mechanical distortions. Additionally, because of the curvature and asymmetry of the side-polished fiber, its optical performance may be polarization dependent, which in many cases is undesirable. Finally, in each of the above-described devices, tuning is accomplished by mechanical movement of one or more components. This increases costs and imposes severe performance limitations, which is highly undesirable.
The paper “Electronically Switchable Bragg Gratings Provide Versatility”, A. Ashmead, Lightwave Magazine, March 2001, describes a VOA structure in which the grating structure is formed entirely within an electrically controllable material. In this case, the waveguide core is covered by an initially liquid matrix of liquid crystal (LC) and monomer. Holographic photo-polymerization is then used to align and create periodically distributed micro droplets of un-disolved LC molecules. This alignment of the LC molecules (along the light propagation direction) creates an effective refractive index nLC of the LC droplets, which matches the refractive index nP of the matrix. This structure is thus optically uniform. However, application of an electric field reorients the LC molecules, which changes the effective refractive index of the LC droplets. This results in the grating structure being “revealed” within the matrix under control of the applied electric field.
This technique suffers a disadvantage in that, in order to reveal the grating at reasonably low voltages, the LC droplets must be relatively large. This intrinsically generates light scattering (e.g. from 5% to 10%) and highly undesirable crosstalk. In addition, the choice of the spectral function of the grating is rather limited, because the alignment of LC molecules (to get nLC=nP) is obtained due to the gradient of polymerization and the formation of periodic planes of micro-droplets of LC molecules. Consequently, only a very limited number of different holographic patterns can be used to expose the monomer solution, without compromising the critical alignment of the LC director. Finally, this method is suitable only for planar guiding structures, because it is difficult to obtain the required LC alignment on a circular geometry guide (fiber).
The paper “Electrically Controllable Long-Period Liquid Crystal Fiber Gratings”, Y. Jeong et al., IEEE Photonics Technology Letters, Vol. 12, N5, p.p. 519–522, May, 2000 proposes a solution in which an electrically controllable LC core fiber grating is used. An electrode having a comb-structure is used to periodically reorient the LC director, which generates an optical grating in the core region. This LC fiber grating has several drawbacks. First, it has high insertion losses due to the junctions between glass and LC core waveguides. Second, the propagation of light in a LC-core guide will inevitably suffer from scattering losses and nonlinearities since light is concentrated in the core area. Finally, current electrode fabrication technology allows the fabrication of only long period gratings.
Accordingly, efficient, electronically controllable optical devices remain highly desirable.